1. Field of the Invention
The present invention relates to a resin-coated ferrite carrier for a two-component electrophotographic developer used for copying machines, printers and the like, its production method, and an electrophotographic developer using the ferrite carrier, and relates in detail to a resin-coated ferrite carrier which is uniform and does not have problems of offensive odors and the like, its production method, and an electrophotographic developer which uses the ferrite carrier, has favorable characteristics in a long period, and can fully respond to the high-speed and full-color.
2. Description of the Related Art
The two-component developer used in electrophotography is constituted of a toner and a carrier; the carrier is mixed and agitated with the toner in a developer box; the toner is given a desired charge; and the charged toner is carried to an electrostatic latent image on a photoreceptor whereby the developer is a carrier material to form a toner image. The carrier is, after having formed the toner image, held by a magnet and stays on a development roll, further returned to the developer box, again mixed and agitated with new toner particles, and repeatedly used in a certain period.
The two-component developer, different from a one-component developer, is one in which the carrier agitates the toner particles, imparts a desired chargeability, and has a function of transporting the toner, has good controllability in developer design, and is therefore widely used in the fields of full-color machines requiring high-quality images and high-speed machines requiring reliability and durability of image sustainability.
In such a two-component electrophotographic developer, an iron-based carrier such as an oxide-coated iron powder and a resin-coated iron powder has been used. Patent Document 1 (Japanese Patent Laid-Open No. 6-19214) describes a carrier for full-color copying machines in which a mixture of an iron powder and a spherical ferrite particles is used as the carrier core material, and the coating resin is composed of two layers with the under coating material of a tetrafluoroethylenic resin containing a polyamideimide resin and the surface coating material of a tetrafluoroethylene-perfluoroalkylvinyl ether copolymer resin.
Since the iron-based carrier as described in Patent Document 1 has a large true specific gravity of one particle and then imparts a large stress on a developing machine, the life-elongation is difficult. Further, the resin-coated carrier composed of two layers raises a problem of layer exfoliation.
Then, ferrite carriers such as Cu—Zn ferrite and Ni—Zn ferrite, which have a lower true specific gravity than the iron-based carrier, are used. These ferrite carriers also have many characteristics advantageous over the conventional iron powder-based carrier in obtaining high-quality images.
As these ferrite carriers, spherical ones are commonly used. However, since the spherical ferrites alone are inferior in the resistance against toner spent, they cannot respond to resent year's higher durability.
Therefore, resin-coated ferrite carriers are used which have the carrier core material of ferrite particles surface-coated with a resin. As the coating resin used here, a silicone resin, a fluorinated epoxy resin and the like are used.
However, resin-coated ferrite carriers coated with a silicone resin have raised problems of fogging in copy images and carrier adhesion due to the change in charge quantity which is caused by internal temperature rise in continuous printing. On the other hand, the resin-coated carriers coated with a fluorinated epoxy resin have raised problems of toner scattering and fogging in copy images due to a decreased charge of the toner spent by continuous printing, and have also shown a faster loss of charge over time, thus inferior in durability. Further, in the case of using the fluorinated epoxy resin, the solvent needs to include an organic solvent with a strong odor such as methyl isobutyl ketone, and this case has problems of offensive odors and the like on production.
Hence, use of a fluororesin as the coating resin is proposed. Patent Document 2 (Japanese Patent Laid-Open No. 55-67754) describes a developer using a carrier whose core is covered with a resinous coating composed of a polytetrafluoroethylene of 5 to 55%, a fluorinated polyethylenepropylene of 5 to 55% and a poly(amide-imide).
However, the carrier core material used in Patent Document 2 is carrier beads, not ferrite particles. Besides, the coating resin used in Patent Document 2 includes a polytetrafluoroethylene, and when the coating resin including a polytetrafluoroethylene is coated on a carrier core material, the coating resin becomes clayey, and does not provide a good coating. Thus, developers using such resin-coated carriers are inferior in various properties.
Patent Document 3 (Japanese Patent Laid-Open No. 54-126040) describes a carrier material for an electrophotographic developer in which a skin layer composed of a material including a fluoropolymer is provided on the surface of a carrier core material via an intermediate layer including a resin which has a lower melting point and a larger dielectric constant than the fluoropolymer. Then, a polyamide resin and an ethylene-vinylacetate resin are exemplified as the intermediate layer.
However, the carrier core material used in Patent Document 3 is steel beads, not ferrite particles (referring to the example) . Further, the resin-coated carrier composed of two layers raises a problem of the interlayer exfoliation, and does not use a polyamideimide resin as a component of the coating resin.
Although various attempts using a resin-coated carrier have been performed in such manners, a resin-coated carrier has not been obtained which provides uniform coating, raises no problem of offensive odor, and exhibits various favorable properties in a long period when made into an electrophotographic developer.
On the other hand, Patent Document 4 (Japanese Patent Laid-Open No. 2002-148869) describes a coating resin layer of a resin-coated carrier which includes a fluororesin, a binder resin and silica particles having a particular specific surface area and a particular average particle size. It states that the silica particles prevent the spent by the toner against the carrier surface through the synergistic action with the fluororesin, and impart a favorable fluidity of the carrier.
However, even in this Patent Document 4, a resin-coated carrier, as in the case described above, has not been obtained which provides uniform coating, raises no problem of offensive odors, and exhibits various favorable properties in a long period when made into an electrophotographic developer.
Therefore, the present invention has on object to provide a resin-coated ferrite carrier having a uniform resin coating and emitting no offensive odors and the like, its production method, and an electrophotographic developer comprises the resin-coated ferrite carrier, is excellent in the charge stability and the image quality stability in a long period, causes less fogging of image or carrier adhesion, has a favorable image density and environmental dependability, and can fully respond to high-speed and full-color imaging.